I do a lot of both, plated and buffet, depending on the facilities, budget, client's perceptions and wants. For the budget jobs, we often do a "plattered" service.
Plattered service, is where we platter the allowed food for each table, on a shared platter. The patrons then pass the platter around amongst themselves. We do have to allow for a little more food than for a plated service, but much less than a stand up buffet.
We can run this for 100 guests, with about 2 chefs, and 4-6 waiters, who will also refill drinks to the table, rather than to each individual. Depending on the number of guests, we can generally run this at a cost of about AUD$50 per person, for 3 courses.